Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
As a wafer is being implanted, it is typically clamped to a chuck. This clamping may be mechanical or electrostatic in nature.
This chuck traditionally consists of a plurality of layers. The top layer, also referred to as the dielectric layer, contacts the wafer, and is made of an electrically insulating or semiconducting material, such as alumina, since it must produce the electrostatic field without creating a short circuit. Methods of creating this electrostatic field are known to those skilled in the art and will not be described herein. For those embodiments using coulombic force, the resistivity of the top layer, which is typically formed using crystalline and amorphous dielectric materials, is typically greater than 1014 Ω-cm. For those embodiments utilizing Johnsen-Rahbeck force, the volume resistivity of the top layer, which is formed from a semiconducting material, is typically in the range of 1010 to 1012 Ω-cm. The term “non-conductive” will be used to describe materials in either of these ranges, and suitable for creating either type of force. The coulombic force can be generated by an alternating voltage (AC) or by a constant voltage (DC) supply.
A second layer, also referred to as the base, may be made from an insulating material. To create the required electrostatic force, a plurality of electrodes may be disposed between the dielectric layer and the insulating layer. In another embodiment, the plurality of electrodes may be embedded in the insulating layer. The plurality of electrodes is constructed from an electrically conductive material, such as metal.
FIG. 1 shows a top view of a chuck, specifically showing the plurality of electrodes 100a-f of the chuck 10. As shown, each of the electrodes 100a-f is electrically isolated from the others. These electrodes 100a-f may be configured such that opposite electrodes have opposite voltages. For example, electrode 100a may have a positive voltage while electrode 100d may have a negative voltage. These voltages typically vary with time to maintain the electrostatic force. For example, as shown in FIG. 1, the voltage applied to each electrode 100a-f may be a square wave. In the embodiment shown in FIG. 1, three pairs of electrodes are employed. Each pair of electrodes is in electrical communication with a respective power source 110a-c, such that one electrode receives the positive output and the other electrode receives the negative output. Each power source 110a-c generates the same square wave output, in terms of period and amplitude. However, each square wave is phase shifted from those adjacent to it. Thus, as shown in FIG. 1, electrode 100a is powered by square wave A, while electrode 100b is powered by square wave B, which has a phase shift of 120° relative to square wave A. Similarly, square wave C is phase shifted 120° from square wave B. These square waves are shown graphically on the power supplies 110a-c of FIG. 1. Of course, other numbers of electrodes may be used.
The voltages applied to the electrodes 100a-f serves to create an electrostatic force, which clamps the workpiece to the chuck.
Ion implantation involves the collision of energetic ions with atoms in the workpiece, which may result in crystal defects and amorphization. This crystalline damage can be restored by thermal processing, known as annealing. However, for certain high dose implants and device structures, typical post-implant annealing may not be sufficient to restore all the damage caused by the implantation. Heating the workpiece during the implant process is known to reduce damage to the workpiece and to preserve more of the crystalline structure to facilitate regrowth during the anneal process.
Workpieces are typically heated by contact, such as through the use of a gas trapped between the workpiece and the chuck, such as when the workpiece is held in place through electrostatic forces. The workpiece may also be directly heated by the chuck. In both embodiments, heat is applied to the lower surface of the workpiece.
These methods may suffer from drawbacks, such as the inability to accurately heat the workpiece, or the ability to quickly and precisely adjust the temperature of the workpiece. For example, in many embodiments, the heat is applied uniformly to the workpiece. However, the ion implantation being performed on the upper surface of the workpiece may impart heat to the workpiece in a non-uniform manner. For example, the ion implantation may heat the center of the workpiece more than the edges. In this case, the added heat applied to the lower surface of the workpiece cannot compensate for this non-uniformity and therefore, the workpiece cannot be held at a uniform temperature. Furthermore, some heating methods rely on the chuck to supply the heat to the workpiece. As the chuck has a thermal mass, which may be significant, rapid changes in applied heat cannot be achieved. Therefore, a system and method of better heating a workpiece would be beneficial.